looks like a willow seedling to me, but i could be wrong. if it's really really strong and almost impossible to pull up, that could be what it is. any willow trees nearby?
I havent got a clue, but if it is willow it is a fantastic thing to have in your field as it is a natural source of aspirin (or potentially another pain killer... but one of them im just having a lack of brain moment...) and being a firm believer that horses are self healers, how much better can you get than a natural source like that in the field!
chaffinch i think you're right. I didn;t know what it was called but definitely seen it before. They die off in the winter and leave you with hollow sticks (great for kindling!). no idea if they're poisonous.